This invention relates to the field of temperature sensing and control and more particularly to an improved differential displacement thermosensing device.
Among the most practical and advantageous thermosensing devices which operate on the differential displacement principle is the rod-and-tube thermostat. This device has a rod positioned concentrically within a tube and rigidly attached to the tube at one end of each. The tube and rod are constituted of dissimilar metals of significantly differing thermal coefficients of expansion so that on change of temperature, the relative displacement between the free end of the tube and the free end of the rod can be utilized for measurement and control purposes.
Rod-and-tube differential displacement thermosensing devices have proved to be practically useful in numerous applications. A relatively new application for which rod-and-tube thermostats are particularly well adapted is in measurement and control of the temperature of catalytic converters or afterburners in automotive exhaust systems. Since the catalyst utilized in these systems is adversely affected by excessive temperatures, it is important to provide systems for both alerting the driver to a high-temperature condition and automatically controlling the exhaust temperature so as to avoid damage to the catalyst before appropriate adjustments or maintenance necessary to eliminate the cause of high temperatures can be performed. The thermal response of the rod-and-tube thermostat probe may be used, for example, as an indication of excessive temperature which can be transmitted to the dashboard for warning the driver, and as a means for initiating automatic control action such as opening a valve to permit flow of outside air over the catalyst to cool it.
In order to provide reliable and sensitive temperature indication and control, the rod and the tube members of the differential displacement thermosensing device should have coefficients of thermal expansion which differ as widely as possible. For fabrication purposes, it is also desirable that the materials from which the device is produced have good workability and be readily welded or brazed to provide for a rigid attachment at positions on each member of the device.
For reliability over an extended period of operation, it is also important that the materials from which the thermosensing device is fabricated be resistant to both corrosion and mechanical deterioration. It is especially important that both the rod and tube of a rod-and-tube thermosensing device be resistant to creep so that the device consistently and repeatably provides the same indication and control function at a given temperature in the environment in which it is utilized. Because of the high temperatures and potentially corrosive atmospheres prevailing in the catalytic converter chamber of an automotive exhaust system, corrosion resistance and creep strength represent demanding and difficult critera for a rod-and-tube thermostat to be used in such a system.